Desalting devices including a tower body and an ion-exchange resin packed therein have been widely used. The tower body has a bulge portion bulging downward from the bottom thereof, and a plurality of strainers are arranged on an inside of the bulge portion. The tower body having the bulge portion at its bottom portion has a high pressure resistance. FIG. 4 and FIGS. 5 and 6 show conventional desalting devices described in FIGS. 7 to 9 of Patent Literature 1.
The desalting device shown in FIG. 4 includes a tower body 1 having a cylindrical straight body portion 1a and a bulge portion 1b formed at a bottom portion of the tower body. A strainer plate 2 is provided at the upper side of the bulge portion 1b, and a plurality of strainers 3 for collecting water are provided on the strainer plate 2. The strainer plate 2 is also formed to have a downward bulging shape. Each strainer 3 has a hollow truncated cone shape, and a large number of slits are provided in its lateral peripheral surface. Each strainer 3 has a foot pipe with a short pipe shape, and the foot pipe extends through the strainer plate 2. The foot pipe is fixed to the strainer plate 2 by means of a nut (not shown) or the like. An ion-exchange resin R is packed at the upper side of the strainer plate 2. Raw water to be treated is passed through the ion-exchange resin R in a downward flow, flows through the strainers 3, and flows into a water collecting chamber 4 at the lower side of the strainer plate 2, and is taken out through an outlet 5.
In the desalting device, the distance L1 from the upper surface of a packed bed of the ion-exchange resin R to each strainer 3 at the outer peripheral side is shorter than the distance L2 from the upper surface of the bed to the strainer 3 at the central portion. Thus, uneven flow occurs in which water flows in a larger amount at the outer peripheral side where the distance from the upper surface of the packed bed of the ion-exchange resin R is smaller than at the central portion. As a result thereof, desalted water flowing through each strainer 3 at the outer peripheral side into the water collecting chamber 4 indicates poor water quality, since the water made contact with the ion-exchange resin R in a short time compared to the average resistance (retention) time. In addition, the ion-exchange resin R at the outer peripheral side causes breakthrough earlier than the resin in the central portion.
FIGS. 5 and 6 show the desalting device of FIGS. 8 and 9 of Patent Literature 1 in which the distance from the upper surface of the ion-exchange resin R to each strainer is made equal to each other. FIG. 6 is a cross-sectional view taken along a VI-VI line of FIG. 5. The ion-exchange resin is not illustrated in FIG. 6.
The desalting device includes a horizontal partition plate 8 at a lower portion thereof, and the strainers 3 are arranged at the upper side of the partition plate 8 such that they are installed on a level with each other. Each strainer 3 is mounted at the lower surface side of a branch pipe 6 for collecting water, and each branch pipe 6 is connected to a water collecting header pipe 7. Desalted water flows through the strainers 3, the branch pipes 6, and the water collecting header pipe 7 in this order, and is taken out through an outlet 7a. 
In this desalting device, the distance from the upper surface of the packed bed of the ion-exchange resin R to each strainer 3 is the same, and uneven flow or shortcut of water to be treated do not occur. A desalting device in which strainers are arranged at the same height is shown also in FIG. 3 of Patent Literature 2.
In such a desalting device in which strainers are arranged at the same height, a space below the strainers 3 is a dead space, and the space within the bulge portion is not used effectively.